The invention relates to a monobloc hollow shaft for transmitting torque, having two end portions which, at their axial outer ends, comprise shaft toothings for introducing torque, and having a central portion which, as compared to the end portions, has a smaller wall thickness. Such shafts, as so-called intermediate shafts, are completed by being provided with two constant velocity universal joints which are slid onto the shaft toothings, and with convoluted boots which seal the universal joints relative to the shaft, such shafts thus turned into side driveshafts.
Monobloc hollow shafts are nowadays normally designed in such a way that they have as constant a polar resistance moment (Wp) as possible across the entire cross-section. The objective is to achieve as low a weight as possible. This was first described in DE 30 09 277 C2.
In DE 21 35 909 A1, it is proposed to provide the rotational resistance moment for a monobloc hollow shaft in the region of the central tube portion in a ratio of 1:3.4 to 1:4.4 lower than in the joint receiving regions. The purpose of this measure is to provide a particularly lightweight shaft.
Nowadays, when a vehicle is starting, such shafts are often subjected to loads beyond the elastic range, with the risk of fracture being immanent. To increase the strength of the shaft, it is common practice to case-harden the monobloc hollow shafts to achieve smaller wall thicknesses with sufficient strength values. It is a well-known fact that case-hardening leads to a relatively brittle fracture behavior with small fracture angles.
The convoluted boots which seal the universal joints relative to the intermediate shaft are, as a rule, fixed on a relatively small diameter in grooves in the intermediate shaft, the disadvantage being that this results in areas of cross-sectional weakness in which, substantially, plastic deformation occurs in the case of load peaks. Such deformation leads to a reduction in the operating ability of the shaft, which, when new load peaks occur, directly results in fracture.
It is, therefore, an object of the present invention to provide a monobloc hollow shaft which is able to withstand repeated load peaks without being subjected to the risk of early fracture. The objective is achieved in that, when torque is introduced via the shaft toothings, due to torsional forces, a central part of the central portion of at least 25% of the length of the shaft is first of all uniformly plastically torsionally deformed.
From DE 40 10 900 C2 there are known hollow shafts wherein the polar resistance moment is not constant along the length of the shaft. But in this case, it is only along half of the shaft length that a relative minimum of the polar resistance moment is provided which, because of the larger diameter, will approximately amount to at least the minimum resistance moment of the shaft ends. Further, in the case of excessive loads, the location of said minimum as well as the above-mentioned grooves will form the location of plastic deformation. This leads to the same disadvantages as mentioned above, i.e., a reduction in the operating ability, which, in turn, will lead to early fracture if new loads occur.
On the other hand, if, in a shaft in accordance with the present invention, the torsional elasticity limit is exceeded, the entire central part of the central portion with a low resistance moment will begin to flow uniformly. As a result, the shaft will not fracture until a large torsional deflection has been reached between the shaft ends under plastic deformation.
In accordance with one embodiment of the present invention, the central part of the central portion with a constant low polar resistance moment comprises at least 25% of the length of the shaft wherein said polar resistance moment of the central part of the central portion is 3-20% lower than said minimum polar resistance moment of the end portions.
In one particular embodiment, the outer diameter of the central portion, including that of the axial ends, is constant and the wall thickness and the diameter of the central part of the central portion are constant.
In another embodiment of the present invention, the regions of transition between the central portion and the end portions constitute steady changes in diameter and wall thickness in order to avoid load peaks of any kind.
In addition, the invention provides for the outer surface of the central portion and of the transition portions to be groove-free in order to ensure that the torsionally softest part is located in the region of the central portion and that no other weakening occurs in the region of the transition portions.
In a further embodiment, the shaft comprises a heat-treated material of uniform strength.
Furthermore, if the shaft of the present invention is used as a driveshaft in the driveline of a motor vehicle, which driveshaft is complemented by universal joints and convoluted boots, the convoluted boots, by means of their respective smaller collar portions, are positioned on the axial outer ends of the central portion without positively engaging same. In this way, it is possible to lengthen the torsionally soft part of the central portion as far as or even underneath the convoluted boots, as a result of which the fullest possible use is made of the scope of the invention.